The present invention relates to a novel energy efficient process for the production of high purity paraxylene (pX) from a feedstock of C8 aromatics which preferably comprises at least about 60 weight percent (wt %) paraxylene wherein a first portion of high purity paraxylene product is obtained in a first crystallization step at a temperature of from about 10xc2x0 F. to about 55xc2x0 F. followed by separation of the washed paraxylene crystals without the need for further reslurry and recrystallization and wherein another portion of the high purity paraxylene product is obtained following a reslurry step that warms crystalline paraxylene obtained from subsequent lower temperature crystallizations to yield a slurry at a temperature of from about 10xc2x0 F. to about 55xc2x0 F. without the need for further refrigeration. The paraxylene crystals are separated from the slurry mother liquor to give high purity paraxylene product. In an alternate embodiment of the invention, the feedstock may comprise at least about 55 weight percent paraxylene.
The separation of xylene isomers has been of particular interest because of the usefulness of para-xylene in the manufacture of terephthalic acid which is used in the manufacture of polyester fabric. Paraxylene is a chemical intermediate useful for the manufacture of terephthalic acid, the major constituent of polyethylene terephthalate. Paraxylene having a purity of at least about 99.5, more preferably of at least about 99.7 weight percent, is used to manufacture terephthalic acid by the oxidation of paraxylene. Other components of the C8 aromatic hydrocarbon feedstream from which para-xylene (pX) is generally produced are ortho-xylene (oX), which is used in the manufacture of phthalic anhydride which is used to make phthalate based plasticizers; meta-xylene (mX), which is used in the manufacture of isophthalic acid used in the production of specialty polyester fibers, paints, and resins; and ethylbenzene (EB) which is used in the manufacture of styrene.
A refinery feedstock of aromatic C8 mixtures containing ethylbenzene and xylenes will typically have the following content:
Equilibrium mixtures of C8 aromatic hydrocarbons generally contain about 22 weight percent para-xylene, about 21 weight percent ortho-xylene, and about 48 weight percent meta-xylene.
Processes to separate xylene isomers include low temperature crystallization, fractional distillation, selective sulfonation with subsequent hydrolysis and selective solvent separation; however, such processes require high operating costs.
Crystallization can be used to separate paraxylene from a C8 aromatic feedstream comprising paraxylene, meta-xylene, ortho-xylene, and ethylbenzene due to the fact that the components have different melting points. Paraxylene freezes at 13xc2x0 C., meta-xylene freezes at xe2x88x9248xc2x0 C., ortho-xylene freezes at xe2x88x9225xc2x0 C., and ethylbenzene freezes at xe2x88x9295xc2x0 C.
Crystallization has been used commercially to isolate and purity paraxylene, typically from a mixture of xylenes and ethylbenzene close to chemical equilibrium. Because of the low concentration of paraxylene in these mixed xylene streams, very low temperatures are generally required to effectively recover the paraxylene from a C8 fraction by crystallization. Furthermore, there is an operational low temperature limit generally taken as the meta-xylene/paraxylene or the ortho-xylene/paraxylene binary eutectic temperature that prevents the complete recovery of all the paraxylene from a C8 fraction. At or below this limit, either meta-xylene or ortho-xylene will co-crystallize with paraxylene. The use of such low temperatures for crystallization is expensive and requires a substantial use of energy. There is a need for a more energy efficient process for crystallizing and purifying paraxylene from a feed containing paraxylene and other C8 aromatics.
U.S. Pat. No. 6,111,161 discloses a process for the production of high purity paraxylene from a charge containing C7-C9 aromatic hydrocarbons in which a first fraction is enriched to at least 30% weight with paraxylene and this fraction is purified by at least one high-temperature crystallization in at least one crystallization zone. Said first fraction is crystallized in a crystallization zone at high temperature T1 and advantageously between +10 and xe2x88x9225xc2x0 C. Crystals in suspension in a mother liquor are recovered, and the crystals are separated from the mother liquor in at least a first separation zone. The crystals obtained are partially melted in at least a zone for partial melting and a suspension of crystals is recovered. The crystals in suspension are separated and washed in at least one separation and washing zone and pure paraxylene crystals and washing liquor are recovered, and pure crystals are optionally completely melted and a liquid stream of melted paraxylene is collected.
U.S. Pat. No. 5,448,005 discloses a process for producing high purity paraxylene from a high weight percent paraxylene feedstock, comprising at least about 70 wt % paraxylene and preferably at least about 80 wt % paraxylene which uses a single temperature crystallization production stage at a temperature in the range of from about 0xc2x0 F. to about 50xc2x0 F. and also uses scavenger stages to raise the paraxylene recovery rate. The single temperature production stage crystallizer of the process employs a wash using only paraxylene product.
The present invention has an advantage over other crystallization processes. It reduces the refrigeration requirements compared to designs disclosed in U.S. Pat. Nos. 6,111,161 and 5,448,005. Thus, it requires less energy expenditure and provides a cost savings compared to those designs. It accomplishes this by separating some or most of the final product early in the separation sequence, thereby reducing the amount of material that requires lower temperature refrigeration. It does not recycle cake back to the first crystallizer from the lower temperature stage(s), but rather uses a reslurry drum to sufficiently warm the crystals so that additional para-xylene product can be recovered without the need for more refrigeration. As calculated according to standard engineering practices, the refrigeration compressor horsepower for the invention can be as much as 13% less than that for comparable designs based on the teachings of U.S. Pat. No. 6,111,161.
The present invention relates to a process for the production of paraxylene from a paraxylene-containing feedstream comprising C8 aromatic hydrocarbons and having a paraxylene concentration of at least about 60 weight percent, the process comprising:
a) crystallizing said feedstream in a first crystallizer at a temperature of from about 10xc2x0 F. to about 55xc2x0 F.;
b) recovering an effluent comprising paraxylene crystals in a mother liquor;
c) separating the paraxylene crystals from the mother liquor in a first separation unit, washing the paraxylene crystals with liquid paraxylene, completely melting the paraxylene crystals, and collecting the liquid paraxylene product;
d) transferring at least a portion of filtrate from the first separation unit to a second crystallizer which is operated at a temperature lower than that of the first crystallizer, crystallizing the filtrate, and recovering an effluent comprising paraxylene crystals in a mother liquor;
e) separating the paraxylene crystals from the mother liquor in a second separation unit and sending the paraxylene crystals to a slurry apparatus;
f) transferring at least a portion of filtrate from the second separation unit to a third crystallizer, which is operated at a temperature lower than that of the second crystallizer, crystallizing the filtrate, and recovering an effluent comprising paraxylene crystals in a mother liquor;
g) separating the paraxylene crystals from the mother liquor in a third separation unit and sending the paraxylene crystals to the slurry apparatus;
h) contacting the paraxylene crystals in the slurry apparatus with paraxylene-containing liquid to form a slurry mixture having a temperature higher than that of the lowest temperature crystallizer;
i) separating the slurry mixture in a fourth separation unit to produce a filtrate and a crystalline paraxylene product, washing the paraxylene crystals with liquid paraxylene, completely melting the paraxylene crystals, and collecting the liquid paraxylene product;
j) recycling at least a portion of filtrate from the fourth separation unit to the second crystallizer; and
k) recycling at least another portion of filtrate selected from the group consisting of filtrate from the first separation unit, filtrate from the fourth separation unit, and filtrate from the first and fourth separation units to the slurry apparatus.
Preferably, the crystalline paraxylene product from step (c) is combined with the crystalline paraxylene product from step (i) prior to melting. This can be suitably accomplished by sending the two paraxylene products to the same melt drum.
The present invention also relates to a process for the production of paraxylene from a paraxylene-containing feedstream comprising C8 aromatic hydrocarbons and having a paraxylene concentration of at least about 60 weight percent, the process comprising:
a) crystallizing said feedstream in a first crystallizer at a temperature of from about 10xc2x0 F. to about 55xc2x0 F.;
b) recovering an effluent comprising paraxylene crystals in a mother liquor;
c) separating the paraxylene crystals from the mother liquor in a first separation unit, washing the paraxylene crystals with liquid paraxylene, completely melting the paraxylene crystals, and collecting liquid paraxylene product;
d) transferring at least a portion of filtrate from the first separation unit to a second crystallizer which is operated at a temperature of from about xe2x88x9210xc2x0 F. to about 35xc2x0 F., crystallizing the filtrate, and recovering an effluent comprising paraxylene crystals in a mother liquor;
e) separating the paraxylene crystals from the mother liquor in a second separation unit and sending the paraxylene crystals to a slurry apparatus;
f) transferring at least a portion of filtrate from the second separation unit to a third crystallizer, which is operated at a temperature of from about xe2x88x9235xc2x0 F. to about 5xc2x0 F., crystallizing the filtrate, and recovering an effluent comprising paraxylene crystals in a mother liquor;
g) separating the paraxylene crystals from the mother liquor in a third separation unit and sending the paraxylene crystals to the slurry apparatus;
h) contacting the paraxylene crystals in the slurry apparatus with paraxylene-containing liquid to form a slurry mixture having a temperature of from about 10xc2x0 F. to about 55xc2x0 F.;
i) separating the slurry mixture in a fourth separation unit to produce a filtrate and a crystalline paraxylene product, washing the paraxylene crystals with liquid paraxylene, completely melting the paraxylene crystals, and withdrawing the liquid paraxylene product;
j) recycling at least a portion of filtrate from the fourth separation unit to the second crystallizer; and
k) recycling at least another portion of filtrate selected from the group consisting of filtrate from the first separation unit, filtrate from the fourth separation unit, and filtrate from the first and fourth separation units to the slurry apparatus.
The present invention additionally relates to a process for the production of paraxylene from a paraxylene-containing feedstream comprising C8 aromatic hydrocarbons and having a paraxylene concentration of at least about 60 weight percent, the process comprising:
a) crystallizing said feedstream in a first crystallizer at a temperature of from about 30xc2x0 F. to about 55xc2x0 F.;
b) recovering an effluent comprising paraxylene crystals in a mother liquor;
c) separating the paraxylene crystals from the mother liquor in a first separation unit, washing the paraxylene crystals with liquid paraxylene, completely melting the paraxylene crystals, and collecting liquid paraxylene product;
d) transferring at least a portion of filtrate from the first separation unit to a second crystallizer which is operated at a temperature of from about 15xc2x0 F. to about 25xc2x0 F., crystallizing the filtrate, and recovering an effluent comprising paraxylene crystals in a mother liquor;
e) separating the paraxylene crystals from the mother liquor in a second separation unit and sending the paraxylene crystals to a slurry apparatus;
f) transferring at least a portion of filtrate from the second separation unit to a third crystallizer which is operated at a temperature of from about xe2x88x9210xc2x0 F. to about xe2x88x925xc2x0 F., crystallizing the filtrate, and recovering an effluent comprising paraxylene crystals in a mother liquor;
g) separating the paraxylene crystals from the mother liquor in a third separation unit and sending the paraxylene crystals to the slurry apparatus;
h) contacting the paraxylene crystals in the slurry apparatus with paraxylene-containing liquid to form a slurry mixture having a temperature of from about 30xc2x0 F. to about 50xc2x0 F.;
i) separating the slurry mixture in a fourth separation unit to produce a filtrate and a crystalline paraxylene product, washing the paraxylene crystals with liquid paraxylene, completely melting the paraxylene crystals, and withdrawing the liquid paraxylene product;
j) recycling at least a portion of filtrate from the fourth separation unit to the second crystallizer; and
k) recycling at least another portion of filtrate selected from the group consisting of filtrate from the first separation unit, filtrate from the fourth separation unit, and filtrate from the first and fourth separation units to the slurry apparatus.
The present invention also relates to a process for the production of paraxylene from a paraxylene-containing feedstream comprising C8 aromatic hydrocarbons and having a paraxylene concentration of at least about 55 weight percent, the process comprising:
a) crystallizing said feedstream in a first crystallizer at a temperature of from about 10xc2x0 F. to about 55xc2x0 F.;
b) recovering an effluent comprising paraxylene crystals in a mother liquor;
c) separating the paraxylene crystals from the mother liquor in a first separation unit, washing the paraxylene crystals with liquid paraxylene, completely melting the paraxylene crystals, and collecting the liquid paraxylene product;
d) transferring at least a portion of filtrate from the first separation unit to a second crystallizer which is operated at a temperature lower than that of the first crystallizer, crystallizing the filtrate, and recovering an effluent comprising paraxylene crystals in a mother liquor;
e) separating the paraxylene crystals from the mother liquor in a second separation unit and sending the paraxylene crystals to a slurry apparatus;
f) transferring at least a portion of filtrate from the second separation unit to a third crystallizer, which is operated at a temperature lower than that of the second crystallizer, crystallizing the filtrate, and recovering an effluent comprising paraxylene crystals in a mother liquor;
g) separating the paraxylene crystals from the mother liquor in a third separation unit and sending the paraxylene crystals to the slurry apparatus;
h) contacting the paraxylene crystals in the slurry apparatus with paraxylene-containing liquid to form a slurry mixture having a temperature higher than that of the lowest temperature crystallizer;
i) separating the slurry mixture in a fourth separation unit to produce a filtrate and a crystalline paraxylene product, washing the paraxylene crystals with liquid paraxylene, completely melting the paraxylene crystals, and collecting the liquid paraxylene product;
j) recycling at least a portion of filtrate from the fourth separation unit to the first crystallizer; and
k) recycling at least another portion of filtrate selected from the group consisting of filtrate from the first separation unit, filtrate from the fourth separation unit, and filtrate from the first and fourth separation units to the slurry apparatus.
The present invention additionally relates to a process for the production of paraxylene from a paraxylene-containing feedstream comprising C8 aromatic hydrocarbons and having a paraxylene concentration of at least about 55 weight percent, the process comprising:
a) crystallizing said feedstream in a first crystallizer at a temperature of from about 10xc2x0 F. to about 55xc2x0 F.;
b) recovering an effluent comprising paraxylene crystals in a mother liquor;
c) separating the paraxylene crystals from the mother liquor in a first separation unit, washing the paraxylene crystals with liquid paraxylene, completely melting the paraxylene crystals, and collecting liquid paraxylene product;
d) transferring at least a portion of filtrate from the first separation unit to a second crystallizer which is operated at a temperature of from about xe2x88x9210xc2x0 F. to about 35xc2x0 F., crystallizing the filtrate, and recovering an effluent comprising paraxylene crystals in a mother liquor;
e) separating the paraxylene crystals from the mother liquor in a second separation unit and sending the paraxylene crystals to a slurry apparatus;
f) transferring at least a portion of filtrate from the second separation unit to a third crystallizer, which is operated at a temperature of from about xe2x88x9235xc2x0 F. to about 5xc2x0 F., crystallizing the filtrate, and recovering an effluent comprising paraxylene crystals in a mother liquor;
g) separating the paraxylene crystals from the mother liquor in a third separation unit and sending the paraxylene crystals to the slurry apparatus;
h) contacting the paraxylene crystals in the slurry apparatus with paraxylene-containing liquid to form a slurry mixture having a temperature of from about 10xc2x0 F. to about 55xc2x0 F.;
i) separating the slurry mixture in a fourth separation unit to produce a filtrate and a crystalline paraxylene product, washing the paraxylene crystals with liquid paraxylene, completely melting the paraxylene crystals, and withdrawing the liquid paraxylene product;
j) recycling at least a portion of filtrate from the fourth separation unit to the first crystallizer; and
k) recycling at least another portion of filtrate selected from the group consisting of filtrate from the first separation unit, filtrate from the fourth separation unit, and filtrate from the first and fourth separation units to the slurry apparatus.
The present invention additionally relates to a process for the production of paraxylene from a paraxylene-containing feedstream comprising C8 aromatic hydrocarbons and having a paraxylene concentration of at least about 55 weight percent, the process comprising:
a) crystallizing said feedstream in a first crystallizer at a temperature of from about 20xc2x0 F. to about 30xc2x0 F.;
b) recovering an effluent comprising paraxylene crystals in a mother liquor;
c) separating the paraxylene crystals from the mother liquor in a first separation unit, washing the paraxylene crystals with liquid paraxylene, completely melting the paraxylene crystals, and collecting liquid paraxylene product;
d) transferring at least a portion of filtrate from the first separation unit to a second crystallizer which is operated at a temperature of from about 5xc2x0 F. to about 15xc2x0 F., crystallizing the filtrate, and recovering an effluent comprising paraxylene crystals in a mother liquor;
e) separating the paraxylene crystals from the mother liquor in a second separation unit and sending the paraxylene crystals to a slurry apparatus;
f) transferring at least a portion of filtrate from the second separation unit to a third crystallizer, which is operated at a temperature of from about xe2x88x9210xc2x0 F. to about xe2x88x925xc2x0 F., crystallizing the filtrate, and recovering an effluent comprising paraxylene crystals in a mother liquor;
g) separating the paraxylene crystals from the mother liquor in a third separation unit and sending the paraxylene crystals to the slurry apparatus;
h) contacting the paraxylene crystals in the slurry apparatus with paraxylene-containing liquid to form a slurry mixture having a temperature of from about 30xc2x0 F. to about 50xc2x0 F.;
i) separating the slurry mixture in a fourth separation unit to produce a filtrate and a crystalline paraxylene product, washing the paraxylene crystals with liquid paraxylene, completely melting the paraxylene crystals, and withdrawing the liquid paraxylene product;
j) recycling at least a portion of filtrate from the fourth separation unit to the first crystallizer; and
k) recycling at least another portion of filtrate selected from the group consisting of filtrate from the first separation unit, filtrate from the fourth separation unit, and filtrate from the first and fourth separation units to the slurry apparatus.
In the process of the invention, the crystalline paraxylene product produced in step (c) is conveniently combined with the crystalline paraxylene product produced in step (i) in a melt drum or other suitable melting means and melted to give a high purity liquid paraxylene product.
A portion of the high purity liquid paraxylene product is preferably used to wash the crystalline paraxylene obtained in steps (c) and (i). The ratio of wash to crystalline paraxylene is suitably about 0.10:1 to about 0.5:1 by weight, more preferably about 0.2:1 to about 0.35:1 by weight. A preferable source of paraxylene for the washing of the purified crystalline paraxylene is the purified liquid paraxylene product produced by the process of this invention.
The slurry mixture formed by mixing the paraxylene crystals obtained from the second and third crystallizations with paraxylene-containing liquid may also be referred to as a reslurry mixture since the paraxylene crystals will have come out of the second and third crystallizers in a slurry with mother liquor prior to separation and are being contacted with paraxylene-containing liquid in a slurry vessel or slurry apparatus to form another slurry. It is preferable to stir or mix the slurry mixture with, for example, a suitable mechanical agitator apparatus. The slurry mixture is maintained in the slurry apparatus (which may also be referred to as a slurry vessel or reslurry drum) for a time sufficient to increase the purity of the crystalline paraxylene contained therein to the desired purity. For the preferred continuous process of this invention, the residence time for the slurry in the slurry vessel is typically about 0.2 to about 2 hours, more preferably about 0.5 to about 1 hour.
Paraxylene produced in the process of the invention has a purity of about 99.5 wt % paraxylene or greater, preferably about 99.7 wt % paraxylene or greater; and most preferably about 99.8 wt % paraxylene or greater.